Environmental regulations restrict emissions of most of the pollutants such as sulfur oxides, nitrogen oxides, mercury and particulate matter. A number of methods have been developed in order to reduce them to within limits permitted by the regulations [Ref. 1-5]. Typically flue gas is treated in three separate steps to reduce NOx, SOx and PM.
Step 1. Reduction of nitrogen oxides: Ammonia gas (NH3) is injected into the flue gas which reacts with NOx as follows:4NO+4NH3+O2→4N2+6H2O  Eq. 12NO2+4NH3+O2→3N2+6H2O  Eq. 2
These reactions require high temperatures around 300 degree C. Addition of catalysts increases the effectiveness of the treatment but increases the complexity of the system, as well.
Step 2. Reduction of particulate matter: PM refers to clusters of mineral matter and unburned fuel that are entrained in the flue gas. PM is typically reduced by an electrostatic precipitator (ESP). ESP creates an electric discharge by applying a high voltage of the order of 10 kV between a wire electrode and a plate shaped collection electrode. The ionized gas imparts electrons to some of the particulates which are forced towards the collection electrode under the influence of the electric field. Particulates lose their charges at the grounded electrode. Those which are heavy enough settle into the collection hopper placed under the electrode.
Step 3. Reduction of sulfur oxides: The flue gas goes through a shower of lime slurry, Ca(OH)2, which converts SO2 to calcium sulfite (CaSO3). Calcium sulfite can be further processed to hydrous calcium sulfate to take advantage of its commercial value.SO2+Ca(OH)2→CaSO3+H2O  Eq. 3CaSO3+2H2O+O→CaSO4.2H2O  Eq. 4
Reduction of mercury: Mercury exists in fossil fuels bound to elements such as sulfur. During the combustion, some of the mercury burns or reacts with other elements in the chamber. Mercury compounds which can be found in the flue gas are mercury oxide (HgO), mercuric chloride or bromide (HgCl2, HgBr2), and mercuric sulfate (HgSO4). The treatment methods described above for NOx, SOx and PM also reduce such mercury compounds. What is emitted at the stack is mostly elemental mercury which can be treated further with additional methods, if necessary.
Prior methods include several multi-pollutant control technologies which use electrical means similar to the current disclosure. These methods will be summarized in order to distinguish what is proposed in this disclosure from the prior art.
Electron beam irradiation (U.S. Pat. No. 5,041,271): This method involves irradiation of the flue gas with high energy electrons while water is added to counteract the rise in temperature. Energetic electrons create oxygen atoms and hydroxyl (OH) radicals which react with NOx and SOx in the gas. The presence of water leads to formation of nitric acid and sulfuric acid. Ammonia is injected into the gas to convert these acids to ammonium nitrate and ammonium sulfate.
Electro-catalytic oxidation (U.S. Pat. No. 5,871,703): Main objective of this process is the oxidation of gaseous pollutants in a barrier discharge reactor. The reactor generates a discharge between two electrodes one of which is covered with a dielectric in order to limit the current flow. A high voltage is necessary to cause the gases to break down to generate electrons. Energetic electrons collide with the oxygen and water molecules to generate oxygen atoms and hydroxyl radicals which oxidize NOx, SOx and mercury pollutants. Oxidized pollutants are collected by a wet ESP placed at the exit.
Plasma enhanced electrostatic precipitator (U.S. Pat. No. 6,365,112): In this method an ESP is modified by including a corona discharge needle at the center of the ESP. A reagent gas suitable for generating a plasma is injected into the ESP through the needle. A typical example of a reagent gas is a mixture of oxygen molecules (O2) and water vapor (H2O). The plasma generates chemically active species including oxygen atoms, ozone (O3) and hydroxyl (OH) radicals. These species react with NON, SON, incompletely burned organics and mercury in the flue gas. The resultant molecules are collected by the ESP.
The common themes in the electrical methods described above are as follows:                Molecules in the flue gas or a reagent gas are ionized by removing electrons and molecular bonds are broken by energetic electrons,        Chemically active species such as oxygen atoms are generated,        Those species react with the pollutants, for example oxidizing them,        Oxidized pollutants are further processed by reacting them with water or other chemicals introduced into the treatment system,        Because a lot of energy is required to remove electrons from neutral molecules and to break up molecules, energy consumption of these methods are high.        